(1) Field of the Invention
The present invention relates to a method of setting electrodes in electrolysis cells, especially setting of carbonaceous anodes in cells producing aluminium by electrolysis according to the Hall-Heroult process.
(2) State of the Prior Art
Aluminium is mostly produced by electrolysis of aluminium oxide dissolved in a cryolite bath. The electrolysis cells allowing this consist of a carbon cathode disposed in a steel shell which on the inside is isolated with refractory materials. Above the carbon cathode is provided a carbon anode or a number of rechangeable carbon anodes which are partly submerged in the cryolite bath and which are gradually reduced by the oxygen originating from the decomposing of the aluminium oxide.
Electric current is led from the top to the bottom of the cells, and the cryolite is kept melted by means of the Joule-effect at a temperature close to the solidification temperature. The most common operating temperatures for these cells lies between 930.degree. and 980.degree. C. The aluminium produced is therefore in a liquid state, and deposits, due to gravity, on the cathode.
The carbon anodes are fixedly attached to so-called anode hangers which are securely held to an anode bar for mechanical and electrical connection. As the carbon anodes are consumed and metal is charged from the cells (the metal represents the actual cathode), the anode bar is lowered to keep a constant distance between the cathode and the carbon anodes.
An electrolysis cell of common size is usually provided with approximately 20 carbon anodes, and since the anodes are consumed gradually, each anode has to be replaced by a new one after 20-24 days. Thus, in each cell a used anode is replaced by a new one every day.
According to the conventional setting method, the new anodes are set or positioned so that the distance from the bottom side of these to the cathodes is the same as the distance for the old ones being exchanged. The exchange of anodes is carried out in different ways. The most common way of doing it is by providing the old (used) anode, or rather the anode hanger, with a chalk mark referring to a reference point on the anode bar, usually the bottom side of the anode bar. The used anode is then placed alongside a new anode on the floor, and the measurement marked with the chalk mark on the old anode is transferred onto the new one, and the new anode is thereafter inserted in the cell.
The above described manual method for setting the anodes is, however, liable to error, caused by the width of the chalk mark, errors of parallax during the transference of the measurements from the old anode to the new anode, irregularities of the surface on which they are placed, etc.
The errors and irregularities result in that the anodes are not positioned at the correct level in the cells, and this will result in unwanted operational disturbances (uneven current absorption, carbon slipping etc.), causing economic losses.
A mechanical device which is based on the conventional method is described in GB patent application No. 2.018.291. The device comprises a crane which is employed to exchange old anodes with new ones. Thus, the old anode is pulled out until, after passing through a certain travel distance, the surface facing the cathode has reached a predetermined horizontal plane. The distance travelled until then is stored. The new anode is positioned with the surface facing the cathode in a second horizontal plane and is lowered towards the cathode in accordance with measurement of the stored level, the distance between the two horizontal planes, and possibly with regard to different saggings of the crane caused by the different weights of the new and old anode.
Even though this mechanical device has eliminated some of the subjective measuring errors, the device is encumbered with objective measuring errors which have influence on the positioning of the anodes. Besides, the above-mentioned device is expensive to produce.
As previously mentioned, incorrect setting of the anodes will give economic operational losses due to disturbances under the electrolysis process. A further disadvantage with the conventional setting method is an increase in anode consumption.